Security brakeman

Hey guys: need a true PR here.

Gemnasium propose a free security analysis from Brakeman, so I fix here all the "HIGH" risks.
In the process, I dropped GroupingController which was used only by TagController (very old concept we didn't maintain) and rewrite a bunch of rspec.

Please let an eye here: any comment is welcome.
`Article#find` will raise an error if the article is not found, so there're no need to check the found article.